Guinea pigs genetically deficient in the fourth component of complement show linkage between the gene controlling C4 synthesis and histocompatibility type. Demonstration of this provides the first animal model of linkage between a complement component in an animal and histocompatibility type. These animals now provide the only animal model for studies of complement synthesis in histocompatibility function. Other studies have shown immunoglobulins of the IgM class and gamma 1 and gamma 3 subclasses of IgG activate the alternative pathway. This was previously thought not to be the case. It was shown that at least half of the IgA myeloma proteins studied did not activate the alternative pathway. Also, this year, studies were continued using the IgG and complement sensitized red cell reagents to characterize and identify various kinds of lymphoid tumors in man. This whole approach has turned out to be of great importance and is leading to a thorough reclassification of human malignancies in collaboration with the NCI. We recently described a glomerular complement receptor. Patients who have immunoglobulin and complement deposition in the glomeruli in vitro, in a subepithelial location, have loss of their glomerulose complement receptor activity; and, therefore, it would appear that this receptor plays an important role in deposition of complement components in the glomeruli.